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Electricity & Radio 271
protecting the rest of the circuit and
saving the trouble of replacing
wiring in odd places.
A more modern replacement is the
circuit breaker, which is a button that
pops out when a fuse would
otherwise break. A trip-free circuit
breaker is one that will trip even if it
is held in.
The voltage regulator is there to stop
the battery being overcharged or the
system being overloaded by the
generator or alternator. There will be
a warning light in the cockpit to
indicate that this is happening, and
that you are getting battery power
only. The over voltage sensor may be
reset with the Master Switch. It may
be tested by turning the ALT half of
the battery switch off for a moment.
When starting, the starter switch will
activate a solenoid, which is just a
bigger switch that can handle more
current, to actually turn the engine.
Since the current is large (60 amps),
there is no fuse protection, which is
why there is a starter light.
Amps
The flow of electrons in a conductor
(i.e. the current) is expressed in terms
of amperes, or amps, which are defined
as the movement of 1 coulomb per
second (a coulomb is the
accumulated charge of a large
number of electrons, actually 6.28 x
1018). In an aircraft, amps would be
measured with an ammeter, or
loadmeter, a useful device for checking
if your battery is being charged. An
ammeter needle should always be
showing in the + side of the gauge,
showing a positive charge.
Volts
The work done to add electrons to
an atom is expressed in volts, which
move from high to low pressure, like
air does. Once a body is charged this
way, it is "pressurised" (for want of a
better word), and the potential
energy is called the potential difference
when it refers to a difference in
energy, or pressure, between two
points. You can look on volts as the
equivalent of water pressure.
Another (older) name for volts is
electromagnetic force, or emf. It is
measured with a voltmeter, which you
might use to check the state of your
battery before starting a jet engine.
Resistance
Even a good conductor slows
electrons down. The longer and
thinner the wire is, the more the
opposition, called resistance, expressed
in ohms. 1 ohm allows 1 amp to flow
when 1 volt is applied.
All this work causes heat, due to the
friction of electrons moving against
each other, and the more work you
make electricity do, the hotter things
get, which is how electric fires work.
If you make it work harder, you get
light as well, hence light bulbs.
When you start using AC, however,
the current flows on the outside of the
cable, increasing the resistance
(many times) because the effective
cross-sectional area is reduced,
which is called the skin effect.
The total resistance is the sum of
resistances in a circuit.
272 Canadian Professional Pilot Studies
Impedance
This is the AC version of resistance,
which arises because an AC circuit
also contains reactance, the opposition
offered to AC by capacitance (below).
Its symbol is Z.
With reactance, energy is always
returned to the circuit and not
dissipated as heat, as it would be
with resistance. However, both are
measured in Ohms.
Capacitance
This is an electrical property that
opposes a change in voltage. A
capacitor can make use of the energy
stored in a magnetic field – it is
made of two conductors separated
by a dielectric, which can be air or
some sort of ceramic. A voltage
across the capacitor will build up a
field between the plates – current
doesn't flow, but a field does. It
stops when the capacitor voltage is
equal to the supply voltage.
If the source of current is taken
away, the voltage stays where it is,
but will gradually leak away over
time, as nothing is perfect, especially
electrical circuits. This is how you
can change the batteries in your
PDA without any other means.
The unit of capacitance is the Farad
(F), named after Faraday. It
represents 1 amp for 1 second with a
change of 1 volt stored as 1 joule of
energy. Factors affecting it are the
size of the plates, the gap between
them and the nature of the dielectric.
The things to remember are that
capacitors will pass AC, but block
DC (as long as there is a charge on
the plates), and they store energy.
Inductance
An electrical property that resists
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